Method for stabilizing and employing temperature sensitive material exhibiting martensitic transistions

ABSTRACT

A method for dimension stabilizing temperature sensitive materials exhibiting martensitic transitions for use in control and work performing devices. The method includes subjecting the martensitic-transition material to a greater unit stress than the material would be required to work against in its application to thereby stretch the material beyond its expected deflection, and subsequently completing a number of temperature cycles while the material is in such overstressed condition, through which it is heated to a point above its transition temperature and cooled back to its annealed temperature. After treatment the material operates through complete work cycles with no loss of dimension stability.

United States att 1191 Willson et al.

[ July 24, 1973 METHOD FOR STABILIZKNG AND 3,558,369 1 1971 Wang et al138/133 EMPLOYING TEMPERATURE SENSITIVE MATERIAL EXHIBITING MARTENSITICPrimary Examiner Richard 0 Dean TRANSISTIONS Attorney-Anthony A. OBrienet a1. [75] Inventors: James R. Wlllson, Trumbull, Conn.;

Donald W. Carey, Anaheim, Calif.

[73] Assignee: Robertshaw Controls Company, [57] ABSTRACT Richmond, Va.A method for dimension stabilizing temperature sensi- [22] plied Sept1,971 tive materials exhibiting martensitic transitions for use [21]Appl. No.: 180,334 in control and work performing devices. The methodincludes subjecting the martensitic-transition material RelatedApphca'mn Data to a greater unit stress than the material would be rel lDivision ofser- 3 1 3 27, 1969, Pat quired to work against in itsapplication to thereby stretch the material beyond its expecteddeflection, and

subsequently completing a number of temperature cy- [52] 148/131 48/115148/13 cles while the material is in such overstressed condi- [51] lllt,Cl. C22! 1/10 tion, through which it is heated to a point above its [58]Fleld of Search 148/13, 13.1, 131, transition temperature and cooledback to its annealed 148/133 3 temperature. After treatment the materialoperates through complete work cycles with no loss of dimen- [56]References Cited sion stability UNITED STATES PATENTS 3,594,239 7/1971Wang 148/13 7 Claims, 5 Drawing Figures IO 18 w 18 L I TM 2o HEAT SOURCEW PAFENIEU 24'975 SHEEI 1 0F 2 FIG. 3

PAIENIEUJULZMBB TEMPERATURE TEMPERATURE sum 2 or 2 ELONGATION METHOD FORSTABILIZING AND EMPLOYING TEMPERATURE SENSITIVE MATERIAL EXHIBITINGMARTENSITIC TRANSISTIONS CROSS-REFERENCE TO RELATED APPLICATION This isa divisional application of pending application Ser. No. 828,243 filedMay 27, 1969, now U. S. Pat. No. 3,652,969.

BACKGROUND OF THE INVENTION The present invention relates generally tothe dimension stabilization of temperature sensitive materials and, moreparticularly, to a method for dimension stabilizing temperaturesensitive materials exhibiting martensitic transitions for use incontrol and work performing devices.

Many diversified applications in the systems control art, to mention butone, require a simple, yet efficient heat sensitive element forconverting thermal energy into mechanical energy. One of the mostobvious applications for such an element is the conventional thermostatused extensively in the control of home and office heating and coolingsystems as well as a number of small home appliances. Heretofore, whatwas considered to be the most effective element for the directconversion of heat into mechanical energy was the bimetallic couplewherein two metals having dissimilar degrees of thermal expansion arebonded together. While such devices have generally served the purpose,they have not proven entirely satisfactory under all conditions ofoperation. Some of the more obvious reasons for these limitations arethe limited mechanical deflection per degree temperature change, theineffieient thermo-mechanical energy conversion, and the difficulty ofmanufacture and standardization.

With recent developments in metallurgy, specifically in the study ofthermally sensitive materials which exhibit martensitic transitions,research efforts have been directed toward seeking a betterthermo-mechanical conversion element. At this point, while a detailedtheoretical explanation of martensitic transition type materials isunnecessary for the purpose of disclosing the present invention, a briefdiscussion thereof will be described for the sake of clarity. Certainnickel-titanium alloys, for example, containing approximately 53.5-56.5percent nickel with the remainder being essentially titanium, have beenfound to undergo a temperature dependent martensitic transition at aparticular critical temperature, this temperature being a function ofthe alloy composition. This transition is produced by applying a load tothe material which is sufficientlygreat to produce a greater deflectionbelow its critical temperature than would normally be expected. Thestructural deformation thus produced causes a molecular change which isaccompanied by the liberation of heat. Graphically it has been foundthat such a structural transition follows a curve of decreasing modulusof elasticity as well as a curve of decreasing modulus of torsion as thetemperature decreases. If the material under stress is now heated to apoint above its critical temperature, it will move in a directionopposite to the direction in which it has been deformed with thecapability to perform useful work. It is important to note, however,that the curves of increasing modulus of elasticity and torsion withincreasing temperature are different than the curves observed during thedecreasing temperature transition; and, more importantly, the cyclictransition produces certain changes in the physical properties of thematerial which cause it to take a set after each cycle preventing itfrom returning precisely to its original position. This periodicallyincreasing offset has, heretofore, proven to be a major inhibitingfactor in the development of an acceptable commerical device using athermo-mechanical element of the type discussed above in the place ofthe conventional bimetallic element.

OBJECTS OF THE INVENTION It is, therefore, an object of the presentinvention to provide a method for dimension stabilizing temperaturesensitive materials exhibiting martensitic transitions for use incontrol devices.

Another object of this invention is to provide a stabilizedthermo-mechanical conversion element having a closedtemperature-deflection loop.

An additional advantage of the present invention is the provision of astabilizing process permitting the use of materials heretoforeimpractical for performing control and work functions where closed-looptemperature-deflection cycles are contemplated.

SUMMARY OF THE INVENTION The present invention is summarized in that amethod for dimension stabilizing a temperature sensitive materialexhibiting a martensitic transition at a critical temperature to performwork upon a load having a particular value includes the steps ofapplying a force to the material having a value greater than theparticular value of the load, and temperature-cycling the materialthrough the critical temperature in a positive and then a negativedirection repetitively in succession to complete a plurality of completecycles.

The inventive concept as well as other objects and advantages of thepresent invention will become more fully apparent from the followingdetailed description of the preferred embodiments of the invention whenconsidered in conjunction with the accompanying drawings. I

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a stabilizing apparatusfor stabilizing a temperature sensitive material for subsequent use in acontrol device;

FIG. 2 shows a temperature sensing control device utilizing apre-stabilized thermal sensor;

FIG. 3 shows a temperature sensing control device including an integralstabilizing means;

FIG. 4 shows a stabilization temperature-elongation curve for thematerial to be used in the apparatus of FIGS. 1, 2 and 3; and

FIG. 5 shows the closed-loop temperature-elongation curves producedafter stabilization by the method and apparatus of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, thereis shown a simplified apparatus operating according to the method of thepresent invention to pre-stabilize a thermally sensitive materialexhibiting a martensitic transition at a critical temperature forsubsequent cyclic use in control devices. The structure consists of acantilevered beam 10 mounted to wall 12 so as to support a load 14 forvertical movement, as shown by the arrow. The load is hung from the beamby a single drawn piece of martensitic transition type wire 16 firmlyattached by connectors 18. A radiant heat source 20, the temperature ofwhich can be manually adjusted, is located adjacent the wire 16 withinheating proximity thereto.

Before going into the details of operation of the device, it isimportant to note first that other suitable heat sources may be employedsuch as direct internal heating by a current flowing therethrough, orthe like; and second, the thermally sensitive material 16 may be shapedand mounted in any number of various ways, for example, as acantilevered beam or a coiled spring, depending upon the operationalcharacteristics desired and the contemplated application of the heatsensing material.

In describing the operation of the device of FIG. 1, reference will bemade to the curves illustrated in FIGS. 4 and 5. It is further pointedout that the description, below, of the operation of the device of FIG.1 will serve also to outline the method of the present invention.

FIG. 4 shows a curve illustrating the elongation charactistics of anickel-titanium wire cycled through its critical temperature a number oftimes. The apparatus of FIG. 1 can be utilized to produce theabovementioned curves, and, in one experiment, a load of 40,000 poundsper square inch was employed. At temperature T, the load applied to thenickel-titanium wire produces an elongation, measured vertically, ofvalue E As the temperature produced by source 20 is decreased, thematerial follows segment A, of the curve which illustrates the rapidincrease of elongation produced by the load when the material passesthrough its I critical temperature. As the temperature is then increasedthrough the critical temperature in a positive going direction, thematerial follows segment A which shows how the alloy tends to return toits original position. This characteristic shape-memory action exhibitedby materials such as nickel-titanium is primarily due to theaforementioned martensitic transitions which take place at the criticaltemperature. As explained above, due to certain molecular changes whichtake place in the structure of the material when temperature cycledunder load, the material does not return precisely to its originalelongation E, but decreases only to point E As the material istemperature-cycled again through its critical temperature, segments Band B of the curve are followed showing a further offset sinceelongation point E is the shortest length the material will then reach.Additional cycle C,-C produces similar results, as expected.

Thus, with a load of 40,000 pounds per square inch, the nickel-titaniumalloy wire when utilized in the apparatus of-FIG. 1 will exhibitmartensitic transitions during temperature cycling through its criticaltemperature but at the end of each cycle will not return precisely toits starting point. If, according to the present invention, the wire,which was temperature stabilized at 40,000 pounds per square inch istemperaturecyclcd with -a reduced load of 20,000 pounds per square inch,for example, the curves illustrated in FIG. 5 will be produced. As canbe seen, the curves form closed-loops since the elongation of the wireat the end of each cycle is precisely the same as at the beginningthereof. Thus, by temperature-cycling a temperature sensitive alloy ofthe type referred to above at a load greater than the load to beutilized in the contemplated control device, the material becomescyclically dimension stabilized and exhibits closed-loop operationrequired in most heat sensing electrical and mechanical control units.

Referring now to FIG. 2, wherein similar numerals are used to refer tosimilar components utilized in FIG. 1, there is illustrated anelectrical single-pole doublethrow temperature sensitive switch 22. Thedevice employs a temperature sensitive element 16. which has beenpreviously dimension-stabilized by cycling at an increased load inapparatus of the type shown in FIG. I. The mid-point of the wire iscoupled to the moveable bar 24, which forms the switchable contact ofthe electrical switch. The bar 24 is in turn connected to a spring load26 which is less than the load 14 utilized during thedimension-stabilizing temperature-cycling process performed by theapparatus of FIG. 1. This assures accurate closed-loop operation asexplained with reference to FIG. 5. The two fixed contacts 28 and 30 ofthe switch 22 are shown afiixed to a base or frame member 32.

One typical application of the apparatus shown in FIG. 2 is aconventional thermostat for a home or office heating system. In thisapplication, radiant heat source 20 schematically illustrates theradiant ambient heat produced by the room or area in which thethermostat is mounted and for which the thermostat is designed tomonitor. As the temperature of the room decreases below the criticaltemperature of the wire sensor 16, thereby indicating a need for heat,the wire is allowed to be stretched by load spring 26 which then movesbar 24 away from contact 28 toward contact 30 completing an electricalcurrent path from contact 30 to the contact on bar 24 to therebyinitiate operation of the heating unit used (not shown). Furthermore, asthe temperature of the room subsequently increases, the temperaturesensing wire 16 returns to its initial position, due to its inherentshape-memory, against the force produced by spring 26 to thereby movebar 24 away from contact 30 back to its original position in physicalcontact with contact point 28. As the temperature of the roomfluctuates, the device will continue to cycle indefinitely in the samemanner, the wire sensing material 16 remaining in its dimensionstabilized condition having once been pre-stabilized according to theprinciples of the present invention.

In FIG. 3, there is shown a more refined switching apparatus combiningthe desired operational characteristics of the alloy under presentdiscussion with the stabilization process of the present invention. Withthis apparatus, an unstabilized nickel-titanium wire, or the like, canbe immediately installed in place as element 16 without requiringpre-cyeling in a separate unit. The switch shown in FIG. 3 is basicallysimilar to the unit in FIG. 2 with the exception of a threadablyengageable load 34 attached to the lower end of spring 26. In operationan unstabilized sensor wire 16 is placed in the switch as shown and theload 34 is threadably removed from its mounting bore 36 in frame 32, asillustrated. In addition, since the additional load will cause a greaterthan normal elongation, contact 30 is bent slightly to the dottedposition shown in FIG. 3 during this stabilization period. Since load 34is now applying an additional force to the wire 16 over that applied byspring 26 above, the-device is ready for pre-cyeling to stabilize thewire sensor for subsequent closed-loop operation without the additionalload 34. To accomplish the pre-cycling heating of element 16, acontrollable electrical power source 38 is shown coupled to the wirewhereupon heat will be internally generated therein at the desiredtimes. The unit is then temperature cycled through a number of completecycles, as typified by the curves of FIG. 4. After this is completed,load 34 is threadably mounted to frame 32 and the contact end of contact30 is bent back to its operative position, whereupon a decreased forcewill be applied to the element 16 which then will provide closed-loopoperation as exemplified by the curves of FIG. 5. It is noted thatradiant heat source 20 and electrical power supply 38 are both providedto apply heat to the temperaturesensitive nickel-titanium element 16;however, other diverse heat sources can be utilized depending upon theparticular installation.

In summary, there is shown and described a method for dimensionstabilizing and employing temperaturesensitive materialsexhibitingmartensitic-transitions at various critical temperatures, suchas nickel-titanium, or the like, for use in control and work performingdevices where accurate temperature-elongation closedloop operation isrequired. Thus, control devices utilizing nickel-titanium, for example,as the temperaturesensing element having new, improved, and desiredcharacteristics are made feasible for many diverse and commerciallyimportant applications.

Inasmuch as the present invention is subject to many variations,modifications and changes in detail, it is intended that all mattercontained in the foregoing description or shown in the accompanyingdrawing shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:

l. A method for dimension stabilizing a member formed of temperaturesensitive material exhibiting a martensitic transition at a criticaltemperature so that said member can subsequently perform work withsubstantially no dimensional change comprising the steps of:

applying to said member a force of a greater unit stress than the memberwould be required to work against in its application to thereby stretchthe member beyond its expected deflection; and

temperature-cycling said member in such overstressed condition bysequentially heating said member to a point above said criticaltransition temperature and then cooling it back to its annealedtemperature repectively in succession for a plurality of completecycles.

2. The method as recited in claim 1 wherein said temperature-cyclingincludes at least three complete cycles.

3. The method as recited in claim 2 wherein said member is formed ofmaterial which comprises an alloy comprising 53.556.5 percent nickel byweight, the remainder being essentially titanium.

4. The method as recited in claim 2 wherein said material comprises analloy comprising approximately 55 percent nickel by weight, theremainder being essentially titanium.

5. The method as recited in claim 1 wherein said member is heated byapplication of external heat thereto.

6. The method as recited in claim 5 wherein said external heat isradiant heat.

7. The method as recited in claim 1 wherein said member is internallyheated by passing an electrical current therethrough.

2. The method as recited in claim 1 wherein said temperature-cyclingincludes at least three complete cycles.
 3. The method as recited inclaim 2 wherein said member is formed of material which comprises analloy comprising 53.5-56.5 percent nickel by weight, the remainder beingessentially titanium.
 4. The method as recited in claim 2 wherein saidmaterial comprises an alloy comprising approximately 55 percent nickelby weight, the remainder being essentially titanium.
 5. The method asrecited in claim 1 wherein said member is heated by application ofexternal heat thereto.
 6. The method as recited in claim 5 wherein saidexternal heat is radiant heat.
 7. The method as recited in claim 1wherein said member is internally heated by passing an electricalcurrent therethrough.